1. Field of the Invention
The present invention relates to a novel lighting circuit for a discharge lamp, which supplies a negative AC voltage to a discharge lamp for ignition to prevent partial leakage of metal ions from the discharge lamp during lighting, thereby preventing a chromaticity change in the light from the discharge lamp.
2. Description of the Related Art
Recently, a compact discharge lamp (e.g., a metal halide lamp) is receiving greater attention as a light source which is a substitute for an incandescent lamp. It is known that a lighting circuit for such a discharge lamp, as adapted to a light source, for example, for a vehicular lamp, includes a DC power supply, a switching power supply circuit, a DC-AC converter, and an igniter circuit positive voltage converter.
A sinusoidal wave or a positive rectangular wave is used as an AC wave which is to be supplied to the discharge lamp.
It is known that when a positive AC voltage is supplied to a discharge lamp to light it, some of metal ions in the discharge lamp leak if there is a metal member (e.g., a reflector or the like) near the discharge lamp. A chromaticity change in the light from the discharge lamp originated from this phenomenon may impair the color rendering property or the like of the discharge lamp, shorten the service life of the lamp or cause other adverse influences.
If the potential around the arc tube of a metal halide lamp is low when metal atoms sealed in this arc tube are ionized at the time of ignition, for example, most of metal ions, though attracted by the silica glass tube which constitutes the arc tube, are shielded by the silica glass tube and do not leak therefrom. However, those metal ions which penetrate the silica glass, like sodium ions, may pass through the tube. This phenomenon is known as Na loss.